In the field of decorative polyolefin-based sheet materials, currently substantially two structures are used. For applications and building components in which the sheet material is subjected to high stretching (for example, >200%) in downstream thermal forming processes, compact film structures which can be made up of several layers are preferably used. These materials generally have a density of >800 kg/m3 at a thickness of 0.5-3.0 mm, as a result of which the building components have a correspondingly high weight and, associated therewith, a high raw material requirement, see also U.S. Pat. No. 6,663,738.
For applications and building components in which the sheet material is subjected to low stretching (for example, <200%) in downstream thermal forming processes, sheet materials having at least one foamed layer, called foam layers, can be used. The compact cover layer can be reduced and constructed in a thickness of 0.2-0.8 mm for a density of >800 kg/m3. The foamed layer is generally constructed having a density of 20-200 kg/m3 and a thickness of 0.5-4.0 mm. The foamed layer reacts elastically to pressure stress, as a result of which pleasant haptic properties in response to pressure of the building components are achieved. Owing to the low density of the foamed layer, the weight of the building components falls, as does the raw material requirement necessary for the production.
Generally, a plastic foamed material based on a polyolefin composition is produced as follows: 5-100% by weight of one or more polyethylene-based plastics (these are understood to include polymers, the weight fraction of ethylene of which is >50% by weight) and also optionally 0-95% by weight of one or more polypropylene-based plastics (these are understood as including polymers, the weight fraction of polypropylene of which is >50% by weight) are mixed with a crosslinking agent and a chemical propellant (foaming agent) and also other process additives such as, for example, lubricants, stabilizers and pigments. A film is produced therefrom, for example, by extrusion. This film is exposed in the downstream process to a source of ionizing radiation, in such a manner that the melt strength is increased by molecular weight build up (crosslinking) in the plastic. In a subsequent heating process, using the propellant, a flat plastic foam material is obtained having a density of 20-200 kg/m3 and a thickness of 0.5-4.0 mm. The foaming process can be carried out vertically in a foam oven or horizontally, for example, in a salt bath. The above described chemically driven foam process leads to fine-celled foams having a very uniform foam cell distribution. Such foams are described, for example, in DE 102005050524 A1.
The resultant plastic foam material can then be joined thermally or by adhesion to surface materials such as, for example, sheet materials based on polyolefins, PVC or polyurethanes, in such a manner that a multi-layer flat material having at least one foamed layer is formed. The surface of the multi-layer flat material can be constructed by an embossing process having a three-dimensional structure, called grain.
The resultant flat material can then be brought into a desired form by processes such as thermoforming, in-mold graining or low-pressure molding. These forms or bodies are used in aircraft, railway vehicles, ships and in motor vehicles, in particular as motor vehicle interior linings or lining components.
U.S. Pat. No. 4,473,665 describes a method in which a plastic foam material based on a polyolefin composition is produced, in that the polymer mixture is loaded above the glass transition temperature with an inert gas at superatmospheric pressure and this gas-loaded melt is then expanded and cooled below the glass transition temperature. According to this method principle, a polyolefin-composition-based plastic foam material having a density of 20 kg/m3-800 kg/m3 may be generated via extrusion or an injection molding process. Sheet materials generated by this principle in the density range <100 kg/m3 generally exhibit a coarse-cell foam structure having a relatively broad cell size distribution.
The foamed polyolefin-based sheet materials generated in this manner are used, inter alia, in the building sector (for example, as footfall sound insulation in laminate floors, heat insulation of pipes, edge trims when laying floors) and in the packaging sector. Owing to the predominantly thermoplastic character of the sheet materials made by this method principle, they are unsuitable to date for the above described thermal forming process for producing three-dimensionally shaped bodies for use, for example, as motor vehicle interior linings or motor vehicle interior lining components.
Further methods for producing multi-layer plastic films are known, for example, from the publications: United States patent application publication 2010/0215934; U.S. Pat. No. 4,832,770; U.S. Pat. No. 4,892,691; EP 413912 B1; United States patent application publication 2009/0022934; JP 2001-096602 A; JP 2005-119274 A; and, U.S. Pat. No. 6,251,319.